Dewatering-related properties of wastewater sludges
No two wastewaters are alike although, in summary, the general effects on filtration of variations in their characteristics are:
- feed compositions are complex mixtures of organic and mineral particles, biosolids, and molecular and ionic substances;
- feed composition is significant in controlling cake resistance, rate of filtration, and cake moisture content;
- feeds invariably require flocculation to “reduce” their fines content, and the negative effect of the fines on filtration;
- due to their higher
The characteristics noted in Table 1 lead to a preference for pressure filters for sludge dewatering; in the case of some industrial sludges the rotary drum filter can sometimes be considered an option. To make sludge feeds more amenable to mechanical dewatering the feed is more often than not pretreated by flocculants or coagulants, agglomerating the feed particles to increase their effective size.
Plate and frame filter presses, recessed plate presses, and membrane plate presses are all used to dewater sludges. Filter plates are supported on side beams (Fig. 4) or suspended from an overhead beam; filter plates of 1.5 m × 1.5 m are typical, but 2 m × 2 m plates are increasingly common—and larger plates are being developed. For wastewater applications, 80 chambers in a recessed plate press or 60 chambers in a membrane plate press is not uncommon.
The ability of membrane plate presses to utilize the
Belt filters are characterised by two continuous, tensioned filter cloths. Flocculated sludge is fed to the lower cloth (belt); initial dewatering is under gravity as the belt carries the sludge into a consolidation zone where it is progressively squeezed under pressure by the upper and lower belts moving towards each other to form a closed “envelope”. The cake is then squeezed under increasing pressure as the cloths move over a sequence of successively smaller diameter rollers. As the two
Rotary drum filters
Vacuum filters have operational and process limitations that can be most important when choosing a filter for sludge dewatering. By definition, the driving force for dewatering is limited by the vacuum that can be applied; in practice, a vacuum of not more than 0.25 bar absolute (−0.75 bar g) can be applied. For this reason vacuum filters are not usually employed in systems where most of the particle sizes are smaller than about 5 μm; in turn, vacuum filters are rarely used to dewater municipal
High solids decanters (Fig. 7) are used to mechanically dewater environmental and biosolids sludges and are often a preferred choice of equipment due to
- the high forces of 2000–4000 g applied directly to the feed solids, enabling lower solids moisture contents (the “ultimate” cake dryness depends on the given sludge);
- its ability to handle higher solids content feeds;
- its continuous operation, with solids throughputs up to about 90 te h−1;
- the solids handling capabilities intrinsic through its design,
Filter cloth choice is a key factor to successful filter performance, and necessary requirements are: (i) good resistance to blinding, as particles become embedded in the cloth filter capacity is reduced and cake moisture increased (this is particularly so in wastewater applications where the composition of the waste stream can vary widely in both its chemical composition and its solids content); (ii) good cake release from the cloth, which is essential to minimise operator intervention; (iii)
Filter media developments
Surface coatings applied to filter fabrics can enhance one or more of its filtration properties; microporous polymer coatings are a relatively new development used to provide a smoother and finer aperture size to the fabric surface, and to facilitate easier detachment of the cake and prolong the lifetime of the medium. A polyurethane coating on a woven polyester substrate is the basis for Madison’s Primapor fabric for use on process filters such as rotary drums and filter presses.
Designs of filtration equipment most suitable for sludge dewatering have evolved to meet the intrinsic characteristics of sludges, the most important of which are their compressibility and fine particle sizes, which lead to cakes with extraordinarily high solids contents close to the filter medium. The sludge feed tends to be networked, that is particles interact strongly one with another to prevent settling and offer a significant resistance to compression, which requires that the forces
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